1 Filed of the Invention
The field of the invention is that of dental compositions. More specifically, the dental compositions developed in the context of the present invention may be used for making dental prostheses and for dental restoration.
2 Related Art
These dental compositions are conventionally epoxy resins, or photopolymerizable silicones or free-radical-polymerizable acrylate resins. These compositions also include particulate reinforcing fillers (e.g. hydrophobicized silica), photoinitiators and optionally photosensitizers or even other functional additives such as pigments or stabilizers.
Once mixed together, these compositions are shaped and then photocrosslinked into a mass whose structure is similar to that of teeth.
The fact that the filler consists of very fine particles (≈0.01 to 5 μm) with a large specific surface area is a factor that limits its degree of incorporation into the resin. The reason for this is that said resin has a limited absorption capacity. As a result, the filler contents of such compositions rarely reach more than 45% by volume. This therefore penalizes the mechanical reinforcing function assigned to the particulate filler.
Furthermore, dental compositions are formulated with reinforcing fillers such as mineral glasses or combustion silicas of very low particle size, the surface silanols and/or residual water of which react with the cationic functions, preventing the compositions from being stored. This is also the case when these glasses or these combustion silicas are pretreated with silanes, for instance glycidyloxypropyltrimethoxysilane or glycidyloxypropyltriethoxysilane, or even methacryloxypropyltrimethoxysilane. Reinforcing fillers interact with the reactive functions of the (photo)polymerizable/crosslinkable species and are thus the cause of instability problems of the dental composition during its storage, which may be up to several months. This phenomenon of instability on storage is accentuated for substantially filled compositions (i.e. overall filler content ≧50%).
U.S. Pat. No. 6,306,926 concerns dental compositions based on epoxy resins (e.g. UVR® 6105, EPON® 828, GY281®), oxetane or vinyl ether, inter alia, which are polymerizable/crosslinkable, cationically and under irradiation, and optionally free-radical-polymerizable (meth)acrylate resins. Besides polymerization inducers such as cationic photoinitiators and optionally free-radical initiators, depending on the case, these compositions comprise a radio-opaque microparticulate mineral filler selected from the following metallic compounds: oxides, halides, borates, phosphates, silicates, carbonates, germanates, tetrafluoroborates, hexafluorophosphates, having an isoelectric point of less than 7. This composition is such that its Barcol hardness is at least 10, after 30 minutes of cationic polymerization at 25° C.
These resins have the drawback of not being fully transparent to the actinic radiation for activation of the polymerization via UV-visible radiation, which is harmful to the reaction kinetics and thus limits the possibilities of obtaining very thick photocrosslinked materials.
Patent application FR-A-2 784 025 is directed toward overcoming this problem by proposing dental compositions based on silicone resins that are polymerizable/crosslinkable, cationically and under irradiation, possibly followed by thermal post-crosslinking. These silicone resins contain oxirane (epoxide, oxetane, etc.) or vinyl ether functionalities. Such compositions comprise:                one or more cationically crosslinkable and/or polymerizable polydimethylsiloxanes bearing on at least one of their ends reactive functions of formula:        
                an effective amount of at least one initiator of onium borate type:        
                at least one photosensitizer, and        at least one dental filler or inert reinforcing filler based on dental glasses or on polymethyl methacrylate or on combustion silica optionally treated with hexamethyldisilazane or polydimethylsiloxane with a specific surface area of 200 m2/g.        
These dental compositions are intended for the manufacture of dental prostheses or appliances and for dental restoration.
These silicones have the advantage over cationically crosslinking organic resins of being highly transparent to UV-visible light and thus of allowing very thick (several millimeters thick) photocrosslinked materials to be obtained in a very short time (less than one minute) with a UV lamp that emits in the visible range >400 nm.
However, these silicones are formulated with reinforcing fillers of Lewis acid or Brönsted acid nature, such as ground glasses or combustion silicas of very low particle size, the surface silanols and/or the residual water of which react with the cationic functions. Such silicone formulations are therefore unstable on storage. Furthermore, when these silicones are formulated with photosensitizers of thioxanthone type, large chromatic variation is observed during exposure for crosslinking purposes. This is reflected by a pinkish coloration of the finished product (after exposure) which is not esthetically desirable.
Besides these problems, these silicone dental compositions remain to be improved as regards increasing the filler content, so as to allow an improvement in the mechanical properties.
Substantially filled dental compositions, presented as having good mechanical properties and comprising from 10% to 70% by volume of filler (e.g. combustion silica) with a particle size (Φm) of between 0.05 and 0.5 μm (less than 50% by volume of particles of diameter Φ>0.50 μm), a free-radical-photopolymerizable acrylic monomer and a dispersant of phosphoric acid ester type of formula:R—[—CO—(CH2)5—O—]nPO3H2 are moreover known, through patent application EP-A-1 050 291.
Such a teaching relating to free-radical dental compositions cannot in any way be transposed to silicone-based cationic dental compositions. The reason for this is that the dispersants R—[—CO—(CH2)5—O—]nPO3H2 are not suitable for cationic compositions, especially since they contain a large acidic residue RPO3H2 that reacts in the presence of oxirane functions and harms the stability of the composition.
It is thus seen that the prior art does not provide a satisfactory solution to the twofold problem of stability on storage of dental compositions based on units that are polymerizable cationically under UV (for example oxiranes) and of dispersion of large amounts of fillers in the resin. Furthermore, the prior art does not provide a solution either to the problem of the residual coloration of dental compositions after crosslinking.